Wide use has been made of an electric turbo charger which is incorporated in an inlet system of an internal combustion engine and configured to compress and turbocharge intake air from an air cleaner in order to enhance the output power of the internal combustion engine.
An electric turbo charger is used which ensures improved acceleration responsiveness by incorporating an electric motor in a rotating shaft as a driving source of the electric turbo charger, thereby controlling revolution of a compressor impeller.
FIG. 10 is a schematic view illustrating the structure of an engine 01 incorporating a conventional electric turbo charger 03. Though the electric turbo charger shown does not have any turbine, an electric supercharger having a turbine exerts the same effect.
The electric turbo charger 03 is disposed upstream of an inlet manifold 012 of the engine 01. The electric turbo charger 03 has an electric motor 04 and a compressor impeller 032 which is disposed in an inlet path and coupled to a rotating shaft 041 of the electric motor 04.
Intake air from a non-illustrated air cleaner is compressed and charged into the engine 01 through the inlet manifold 012 by the compressor impeller 032 driven by the electric motor 04.
The rotating shaft 041 of the electric motor 04 is rotatably supported by a pair of bearing support portions (not shown) placed in a housing 031 via bearings 042. A motor rotor 043 configured to rotate the compressor impeller 032 is disposed on an intermediate portion of the rotating shaft 041 coupled to the bearings 042 and the compressor impeller 032.
A stator 044 for rotating the motor rotor 043 by generating a magnetic field is placed in the housing 031 at a position opposed to the motor rotor 043.
Reference numeral 06 designates an engine control unit (ECU) which controls operation of the engine 01 while controlling operation of the electric turbo charger 03 by controlling the amount of current to pass through the stator 044 in accordance with operating conditions of the engine 01 by means of a power transducer 08. Reference numeral 07 designates a battery which is a power source for the power transducer 08.
FIG. 11 is a schematic view illustrating the motor rotor 043 of the electric motor 04 according to Japanese Patent Application Laid-open No. 2000-145468 (Patent Document 1) as a representative example of a conventional technique. The motor rotor 043 according to Patent Document 1 is provided with a pair of bearings 042 on the rotating shaft 041 having one end fitted with a turbine blade 05 (equivalent to the compressor impeller 032 shown in FIG. 7), the bearings 042 being spaced apart from each other on the rear side of the turbine blade 05.
A rotor 046 is disposed between the pair of bearings 042.
In this case, it is essential that a permanent magnet be used in the rotor 046 and that a sleeve 047 be fitted over the outer periphery of the permanent magnet in order to prevent the permanent magnet from scattering to the periphery.
The stator 044 for rotating the rotating shaft 041 in cooperation with the permanent magnet of the motor rotor 043 is disposed in the housing 031 so as to circumscribe the outer periphery of the motor rotor 043.    Patent Document 1: Japanese Patent Application Laid-open No. 2000-145468
However, the temperature of the place where the motor is disposed becomes elevated due to self-heating of the motor and heat generated from the engine and, hence, the motor using the permanent magnet is demagnetized with rising temperature, thus exhibiting substantial degradation in performance.
As a remedy, there is a magnetic inductor type motor as one of motors of the type which does not use any permanent magnet in the motor rotor.
The magnetic inductor type motor is configured to drive a rotor core which is a rotor comprising stacked electromagnetic steel sheets or a combination of stacked electromagnetic steel sheets and an iron material by a stator circumscribing the rotor core. A field flux is generated by a field coil provided along the stator axis or a field magnet generating a magnetic flux along the stator axis.
However, the supercharger has a structure in which the magnetic inductor type motor and the compressor impeller are mounted on a shaft which is rotatably supported in a housing by means of bearings.
The rotor core mounted on the rotating shaft of the magnetic inductor type motor comprises thin electromagnetic steel sheets which are stacked in the thickness direction of the steel sheets and are securely fixed to the rotating shaft.
Thorough control of maintenance of the assembly quality is needed to prevent the thin electromagnetic steel sheets from exfoliating and deforming during the operation of fixing the stacked electromagnetic steel sheets to the shaft of the turbine rotor, thus resulting in a problematic increase in cost with increasing man-hour.
There are a case where the bearings are positioned at opposite outer ends of the rotor core and a case where the bearings are positioned only between the compressor impeller and the rotor core.
In such cases, an assembly operation includes press-fitting or shrink-fitting (or cooling-fitting) the compressor impeller and the rotor core over the shaft after balance adjustment has been made to each of the compressor impeller and the rotor core as a single item. For this reason, when the fitting of the rotor core and compressor impeller ends in failure, the compressor impeller, the shaft, the bearings which have been already mounted, and the like cannot be used any longer.
Further, since the rotor core mounted on the rotating shaft of the magnetic inductor type motor comprises the thin electromagnetic steel sheets in a state of being stacked in the direction of the thickness thereof, variations in the interlayer gaps between the stacked electromagnetic steel sheets are likely to give rise to a difference in magnetic property between individual rotor cores, thus resulting in a problem that a stable control of quality maintenance becomes difficult in terms of motor performance.